1. Field of the Invention
This invention relates to video signal interpolation. More particularly, the invention relates to methods of and apparatus for deriving vectors for use in the interpolation of a video signal, and to interpolators for spatially interpolating a video signal. The invention also relates to interpolator apparatus for interpolating fiels or frames of an input video signal to form interpolated fields or frames, and to motion compensated standards converters.
2. Description of the Prior Art
Requirements exist for converting an interlace scanned video signal into a progressive scan (sometimes called sequentially scanned) video signal, in other words to convert interlace scanned fields of a video signal, each of which consists of the odd or even scan lines only from a frame of the video signal, into progressive scan fields of the video signal each of which consists of a full set of both odd and even scan lines, by a process of spatial interpolation. Examples of where such a requirement arises are where a picture is to be displayed in slow motion, and where it is required to improve the performance of a video display monitor.
The problems inherent in such conversion will be briefly explained with reference to FIG. 1. This shows diagrammatically an overlay of two images of a moving object 1 in two successive fields of an interlace scanned video signal. The portions of the object 1 present in the respective fields are indicated by the bold portions of the scan lines. In this case, neither simple vertical nor temporal interpolation will give a satisfactory result. Thus, vertical interpolation will result in vertical alias, whilst temporal interpolation will result in a double image.
More generally, the primary reason why standard interpolation does not give satisfactory results is that the vertical sampling inherent in the generation of a field of video does not follow classical sampling theory, and results in considerable vertical alias. This problem can be alleviated to some extent by vertical filtering before sampling, but this introduces a significant loss of resolution in normal picture sequences.
It has therefore been proposed to use previous field information where a motion detector indicates low levels of picture movement, or alternatively to use some form of image processing to create the missing lines. The first of these techniques tends to fail with some picture sequences, such as sports scenes, where it is likely that the focus of attention is on a moving object. In such a case it is the movement of the object which is of interest, and hence the information available from the previous field is of no use. The second technique is therefore to be preferred, used either alone or in combination with the first technique, but previously proposed versions of the second technique have been found to be subject to considerable error with some types of picture.
Furthermore, video standards converters are well known devices used to convert video signals from one standard to another, for example, from a 625 lines per frame, 50 fields per second standard to a 525 lines per frame, 60 fields per second standard. Like spatial interpolation, video standards conversion cannot be achieved satisfactorily merely by using simple linear interpolation techniques, because of the temporal and vertical alias which is present in a video signal. Thus, simple linear interpolation produces unwanted artifacts in the resulting picture, in particular, the pictures are blurred vertically and jitter temporally.
To reduce these problems it has been proposed that video standards converters should use adaptive techniques to switch the parameters of a linear interpolator in dependence on the degree of movement in the picture represented by the incoming video signal.
It has also been proposed, for example, for the purpose of data reduction in video signal processing, to generate motion vectors from an incoming video signal by a block matching technique, in which the content of a search block in one field or frame is compared with the respective contents of a plurality of search blocks comprising a search area in the following field or frame, to determine the minimum difference between the contents so compared, and hence the direction and distance of motion (if any) of the content of the original search block.
The present invention is therefore also concerned with the extension of the above-mentioned methods and apparatus to motion compensated interpolation of fields or frames, and to motion compensated standards converters.